This invention relates to MOS integrated circuits, and in particular to a contact structure which permits fabrication of such circuits with shallow junctions.
In the present development of MOS integrated circuits, a primary objective is to produce junctions as shallow as possible which permits fast operation and little side wall capacitance. Shallow junctions, however, result in contacting difficulties. In particular, when aluminum is utilized to contact the surface region, the sintering step causes aluminum to alloy with the semiconductor at a depth past the junction resulting in spikes which can short the junction. Several solutions have been suggested to eliminate this spiking problem. A typical approach when fabricating circuits with As-doped surface regions, is to diffuse phosphorous in an area of the exposed surface region to a depth below the surface region. Thus, during a later sintering step, the aluminum will not penetrate through the deeper phosphorous diffusion. Further, use of an aluminum-silicon alloy in place of pure aluminum as the contact metal will reduce the depth of alloying and prevent spiking, either alone or in combination with the phosphorous diffusion step. (See, e.g., U.S. Pat. No. 3,609,470 issued to Kuiper.) It has also been recognized that spiking can be prevented by depositing a thin layer of silicon over the contact windows followed by the aluminum deposition. (See, e.g., U.S. Pat. No. 3,740,835, issued to Duncan.) In such cases, the deposited layer of silicon, rather than the underlying semiconductor body, will act as a source for diffusion of Si into the Al layer.
A further problem in achieving shallow junctions involves necessary heat treatments in formation of contact windows. In a typical process, subsequent to formation of the surface regions, a phosphosilicate glass is deposited over the semiconductor body and contact windows defined therein by photolithography. The resulting side walls are steep with sharp edges and do not permit complete coverage by the contact metal. The glass is therefore fire polished by heating at a temperature sufficient to soften the glass and slope the side walls of the windows. This typically requires a temperature of approximately 1000.degree. C. to 1100.degree. C. for 15 minutes, which causes a diffusion of the surface region impurities further into the semiconductor body increasing the junction depth.
In general, it is desirable to reduce the extent of heating during the processing of MOS integrated circuits so that the junctions can be kept as shallow as possible. It is, therefore, a primary object of the invention to provide a contact structure and method of fabricating such structure which reduces the number of heating steps and still provides reliable ohmic contact to the devices in the circuit.